Is there any reason why this hasn't been done? It is certainly a strange business method of acquiring intellectual property for the sole purpose of locating and suing unknowing infringers.

Perhaps you can only patent the method that you use to find the infringers? Maybe you can patent a seemingly-specific way of searching the web for potential infringers?

I have a poor understanding of what can and cannot be patented. It seems like everything can be patented, but then it needs to be defended in court if it's challenged.